1. Field of the Invention
The present invention relates to a method and apparatus for forming an annular member wherein axially opposed ends of the annular member are formed simultaneously. One end is enlarged in diameter and the other is reduced in diameter. This forming method is also available for manufacturing automobile wheel disks from cylindrical members. Such cylinders are formed by rounding a flat plate and butt-welding along the longitudinal seam.
2. Description of the Prior Art
Various forming methods for enlarging or reducing a single end of a tube are disclosed in, for example, "Tube Forming Techniques" edited by Tube Forming Group of Japan Plastic Forming Engineering Association (published August 1986) and "Method for Forming a Pipe End and Recent Trend Therefor", Pages 18-24, Vol. 26-3 of "Press Techniques".
These prior art forming methods are summarized in FIGS. 3-10. More particularly, FIG. 3 illustrates a press-forming method wherein a pipe 22 is pushed against a mandrel 24 so that an end portion of the pipe is enlarged in diameter. FIG. 4 illustrates a spinning method wherein a spinning roll 28 is pushed against a pipe 26 so that the pipe is reduced to a diameter of the rotating mandrel 30. FIG. 5 illustrates a roller-forming method wherein a pipe 32 is pushed against a mandrel 34 having rollers 36 so that an end of the pipe is enlarged in diameter. FIGS. 6 and 7 illustrate a swaging method wherein a pair of dies 40 are repeatedly pushed against a pipe 38 in a direction perpendicular to the longitudinal axis of the pipe. After each stroke of the disks 40, they are turned incrementally around the pipe 38.
Finally, FIGS. 8 through 10 illustrate a method for forming a pipe end by using an oscillating-type die. More particularly, as illustrated in FIG. 8, a die 42 having an axis inclining with respect to a predetermined axis is allowed to freely rotate about the inclining axis. During formation, as illustrated in FIG. 9, the pipe 44 is chucked by a chuck 48 and is pushed against the inclining die 46 (which corresponds to the die of FIG. 8). The die 46 is forcibly rotated about the inclining axis by the rotation of the chuck 48 resulting in diameter enlargement of the pipe end. When a pipe end is reduced in diameter, as illustrated in FIG. 10, a pipe 44 is chucked and rotated by a chuck 54 and is pushed by a die 52 having an axis inclining with respect to the pipe axis.
However, there are several problems with the conventional forming methods.
First, with any forming method other than the press-forming method, the pipe ends cannot be formed simultaneously because one end must be chucked, thus preventing access of such end to the forming dies. In order to form both ends of the pipe, two sequential stages are necessary. Further, after the one end has been formed, the formed end must be chucked by a specially configured chuck when the other end is formed. Thus, changing configurations is not easy. Furthermore, in spinning, a spiral pattern and steps are generated in the surface of the formed member and a step portion is formed in the surface.
Second, the press-forming method requires a very large forming load because the entire circumference of the end of the pipe is formed at the same time as compared with the forming load needed in the methods depicted in FIGS. 9 and 10 wherein the end of the pipe is formed partially and proceeds about the circumference of the annular member. As a result, a large capacity of the forming machine is necessary, which is accompanied by an increase in the equipment cost. Further, the press forming method cannot be used for thickness reduction forming of steel pipe because too much load is necessary. Such a thickness-reduction forming will require heating the pipe to decrease pipe rigidity. The heating will require a scale-deleting step and will increase the production cost.